A magnetic probe, sometimes called a magnetic pickup or magnetic sensor, is a non-contact transducer employed to convert the kinetic energy of a ferromagnetic element moving past the probe into electrical energy. These devices are used in a variety of applications, frequently being employed to sense the movement of gear teeth or similar elements for synchronization and timing purposes. A common construction for a magnetic probe of this general type comprises a permanent magnet of disc or rod shape having a pole piece of reduced diameter projecting coaxially from the disc and an electrical sensing coil mounted on the pole piece. The magnet, pole piece, and coil are usually mounted in a cylindrical housing, though the housing is sometimes omitted.
Magnetic probes are highly advantageous in many applications, as compared with mechanical or optical sensors. There are no switch contacts to wear out, no cables to break, no mechanical couplings subject to failure, no lamps to fail, and no necessity for protection from dirty environments. Magnetic probes are also highly reliable and afford extremely long operating life with little or no maintenance cost.
A magnetic probe of conventional construction, as generally described above and as illustrated in FIG. 1, however, does present some substantial technical problems and difficulties. Thus, some form of non-magnetic housing is a virtual necessity for the probe, because a magnetic housing or other magnetic member in close proximity to the permanent magnet may effectively short-circuit the magnetic sensing field of the device and prevent effective operation. Indeed, magnetic losses in conventional probes are quite high, and it is often difficult to obtain an output signal of adequate amplitude. The mounting of the probe is often critical, particularly if the probe is used in a magnetic environment as in a timing device for an internal combustion engine. For applications of that kind, the probe housing must usually be constructed of stainless steel or other non-magnetic steel, materially increasing the cost of the probe. Furthermore, if located within a substantial magnetic field the conventional probe may be de-magnetized and thus rendered inoperative.
A form of magnetic transducer, which could be used as a probe, is shown in Palazzetti U.S. Pat. No. 3,942,045. One form of the Palazzetti transducer employs a cylindrical permanent magnet, uniformly magnetized in a radial direction, in a magnetic circuit structure that permits the use of a housing formed of steel or other magnetic material, thus avoiding one of the principal problems of the conventional probe discussed above. But this form of the device incorporates magnetic structures of undesirable complexity and cost, and does not provide strong enough output signals when reduced to the size required for some critical applications.